The seminars listed here can be given upon request. A biographical sketch can be found at the
bottom of this page.
Tutorial: The Art of Science
The current model for training researchers is very
much like the medieval system where an apprentice follows a master for
years of training. This model gives graduate students valuable hands-on
experience. What often lacks in this educational model is an explicit
transfer of skills and information at a rate and moment in time that is
effective for acquiring research skills in a timely manner. For this
reason I started the course "The Art of Science" at the Colorado School
of Mines. I teach this material in different forms; as a semester-long
course, as a short course of a few afternoons, or as a single one-hour
seminar. Depending on the length of the course and the wishes of the
audience I choose from the following topics:
Tutorial: seismic interferometry, who needs a
seismic source? (download ppt)
Seismic interferometry is a technique for imaging
without coherent sources. The idea is to combine waveforms, generated
by ambient noise, that are recorded at different receivers in a way to
provide the waves that would propagate between these receivers as if
there was a source at one of these receivers. This obviates the need to
have a soure located at one of the receivers. In the tutorial I cover
different formulations of the theory that explain seismic
interferometry, and present examples with field data that show the
possibilities that are opened up with this new technique. With the
advent of permanent networks of seismometers in exploration seismology
and global seismology, seismic interferometry opens up new methods for
imaging and monitoring.
Title: Extraction of the Green's function from
ambient fluctuations for general linear systems
Title: Extracting the building response from
Structures such as buildings or bridges are often
instrumented with acellerometers to monitor the vibrations. Since the
excitation of these structures usually is incoherent, these recordings
do not directly give the impulse response (the response to an impulsive
loading) of these structures. I show how seismic interferometry can be
used to extract the impulse response from a building from incoherent
vibrations recorded in a building after an earthquake. I also show that
depending on the data-processing that is applied, either the
propagating waves or the normal modes of the buliding can be retrieved.
With this apprach the response of the building can be separated from
the coupling of the building to the subsurface. In this seminar I show
the theory and apply this to the motion recorded in the Millikan Libary
in Pasadena (California).
Multiple scattered waves are not very useful for deterministic imaging in complicated media because there is no known algorithm to construct such an image. Because multiple scattered waves have long wave-paths, these waves are very sensitive to small changes in the medium. Coda wave interferometry is a new technique that can be used to detect minute changes in a strongly scattering medium using changes in the multiple scattered waves over time. This technique is analogous to speckle pattern interferometry as used in optics, but takes advantage of the phase information in recorded waves. Because of its modest hardware requirements, coda wave interometry has a large number of applications. These include geotechnical applications (dam-monitoring, tunnel monitoring), the evaluation of hazards (volcano and fault monitoring), non-destructive testing, locating earthquakes, and monitoring of hydrocarbon reservoirs.
Title: Time-reversed imaging as a diagnostic of wave and particle chaos
Chaotic behaviour of particles concerns the stability properties of trajectories under perturbations of initial conditions. For waves, chaotic behaviour is less clearly defined. Both Newton's law and the Helmholtz equation are symmetric under time-reversal. This means that particles or waves emitted by a source at t=0 should refocus on the source when their propagation is reversed in time. Chaotic behaviour will prevent this to occur. This idea is tested for a system of very strong scatterers through which particles and wave propagate. Analytical expressions are derived for the critical perturbations of the initial conditions of both waves and particles. It is shown that the resulting behaviour of waves and particles are fundamentally different with critical length scales ranging over 15 orders of magnitude. The analytical results are illustrated and confirmed by numerical simulations.
Title: The arrow of time
It is great irony of science that the most
fundamental concepts are often most dificult to understand. The concept
"time" is an important example of this. The laws that describe the
basic forces in nature are symmetric for time reversal. This means that
they do not change when one changes the direction of time by replacing
the time t by -t. However, this clearly contradicts our experience; we
perceive a direction of time. This direction is called the "arrow of
time." Different arrows of time can be distinguished: the thermodynamic
arrow of time, the biological arrow of time, the radiative arrow of
time, the mechanical arrow of time and the cosmological arrow of time.
The relation with natural laws that are not invariant for time reversal
is discussed and some pitfalls are shown in "deriving" equations with a
direction of time from the fundamental laws of physics. The symmetry of
time reversal has important applications in geophysics, examples are
shown of this. The final question remains: "what explains the arrow of
time that seems to pervade our daily experience?"
Title: Earthquake prediction, a political problem?
Summary: Earthquakes are among natural hazards that
threaten society. For this reason earthquake prediction is a
field of research that arouses considerable nterest. An overview of the
earthquake prediction problem is given. It will be
shown that the earthquake-prediction activities of scientists confront decision-makers with a fundamental trade-off between information and probability that the eartquake indeed occurs. However, this does not imply that scientists cannot contribute to alleviate the danger posed by earthquakes.
Roel Snieder holds the Keck Foundation Endowed Chair of Basic Exploration Science at the Colorado School of Mines. He received in 1984 a Masters degree in Geophysical Fluid Dynamics from Princeton University, and in 1987 a Ph.D. in seismology from Utrecht University. In 1993 he was appointed as professor of seismology at Utrecht University, where from 1997-2000 he was appointed as Dean of the Faculty of Earth Sciences. In 1997 he was a visiting professor at the Center for Wave Phenomena. Roel served on the editorial boards of Geophysical Journal International, Inverse Problems, Reviews of Geophysics, the Journal of the Acoustical Society of America, and the European Journal of Physics. In 2000 he was elected as Fellow of the American Geophysical Union for important contributions to geophysical inverse theory, seismic tomography, and the theory of surface waves. He is author of the textbooks "A Guided Tour of Mathematical Methods for the Physical Sciences" and "The Art of Being a Scientist" that are published by Cambridge University Press. Roel is a foreign member of the Royal Netherlands Academy of Arts and Sciences. In 2011 he was elected as Honorary Member of the Society of Exploration Geophysicists. Since 2000 he is a firefighter in Genesee Fire Rescue.